A conventional field-emission cathode has been proposed on pages 5248 to 5263 of "Journal of Applied Physics, Vol. 47, No. 12, 1976" by C. A. Spindt et al. The conventional field-emission cathode comprises a silicon substrate, an insulating layer of 1 .mu.m thickness formed on the silicon substrate, a gate electrode of molybdenum formed on the insulating layer, cone-shaped emitters of molybdenum having a height of 1 .mu.m provided in cavities of 1.5 .mu.m diameter formed through the insulating layer and the gate electrode, wherein the emitters are ohmic-contacted with the silicon substrate. In the field-emission cathode, the emitters are arranged in a predetermined pattern.
In operation, a voltage of several tens to 200 V is applied across the gate electrode and the silicon substrate, such that the gate electrode is at a positive potential to generate an electric field of more than 10.sup.7 V/cm at the tips of the emitters. Thus, electrons are emitted from the tip of the emitters.
In the field-emission cathode, emitted electrons of more than 100 .mu.A are observed per one emitter to make it possible that it can be applied to a variety of applications. For instance, the fabrication of a switching device using a triode comprising the field-emission cathode as an electron source, and a display panel in which fluorescent spots are illuminated by electrons emitted from a planar electron source having matrix-arranged field-emission cathodes have been conducted. In this regard, the Japanese Patent Kokai No. 48-90467 discloses a color picture tube having an electron emission source structured by one or matrix-arranged field-emission cathodes in place of conventional thermal cathodes.
In such an electron emission source, a bonding wire or a bridge-shaped conductive member is ohmic-contacted to a gate electrode, so that a voltage is applied across the gate electrode and a substrate.
In an electron beam-application device or machine using such an electron source, it is required that an electric field (equipotential surface) generated above the electron source is vertical to the electron source and axis-symmetrical relative to a normal extending through an electron emitting center to focus or control the emitted electrons. Therefore, the bonding wire must have a sufficient distance from an electron emission surface, or must be shielded by a conductive member, so that the electric field is not disturbed.
Such a structure makes a gate electrode and an electron source large, so as to lower design freedom of the electron source. Even worse, making the gate electrode large makes a capacitance between the gate electrode and the substrate, so as to hinder high-frequency operation. Further, the bonding wire is bonded at both ends to the gate electrode and a power supply potential area, respectively so that reliability is lowered in regard to thermal impact and vibration shock.
In forming a contact-hole communicating from the top surface to the bottom surface of a substrate for a gate electrode to be pulled on the back surface thereof opposite to an electron emission plane, an ion-milling process, or a reactive-ion etching process, etc. is used. However, these processes taken a long time. In addition, the process is difficult in that a fine and thin conductive member is deposited on an inner wall of the contact-hole.